A variety of vacuum pump types are available that are capable of achieving ultra-high vacuum (UHV, around 10−9 Torr or lower). Historically the pump of choice was the oil diffusion pump, which uses conical streams of oil to impart momentum on gas molecules.
One of the most popular pump types used today is the turbomolecular pump, which uses fast-spinning blades to impart momentum to gas molecules to maintain a pressure differential. While such pumps provide an oil-free solution for reaching UHV, as they are mechanical devices they can suffer mechanical failures. They also can create noise and vibration that can have a negative impact on a vacuum process.
Pumps that ionize gas and trap the ionized gas on surfaces within the pump, e.g. sputter ion pumps (SIPs) are also available. While these are also oil-free and additionally are non-mechanical, they suffer from saturation effects, and the sputtering away of material can limit their lifetime by creating holes in cathode plates and causing electrical shorts when portions of the cathode break away. These problems are particularly acute when operating at higher pressures (e.g. greater than 10−6 Torr).
Further examples of pumps that ionize gas are described by Haine, “Improvements relating to high vacuum pumps,” UK Patent No. 684710 (1952), and Farnsworth, “Vacuum Pump,” Canadian. Patent No. 728281 (1966).
There is an ongoing need for vacuum pumps capable of achieving UHV while addressing the problems conventionally associated with mechanical pumps such as turbomolecular pumps and ion pumps such as SIPs.